TRILL ISIS: Fix incorrectly equal ett variables for different subtrees

Change-Id: I38d886d50d74d75a900274df8bc24114e84926e7
Reviewed-on: https://code.wireshark.org/review/3744
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>